Bacterial infections are in most instances successfully treated by administration of antibiotics to patients in need thereof. However, due to careless or thoughtless use of powerful antibiotics, many pathological germs become resistant against antibiotics over time. One threatening example is Klebsiella pneumoniae. 
The genus Klebsiella belongs to the family Enterobacteriaceae and is divided into at least 4 species. They are gram-negative, capsulated, oxidase-negative, non-motile, straight rods. They are facultative anaerobes, having both a respiratory and fermentative metabolism. Most strains can use citrate and glucose as their sole carbon source. Some strains can fix nitrogen. They are commonly found in the intestines, clinical samples, soil, water and grains. The species Klebsiella pneumoniae can be divided into 3 subspecies; pneumoniae, ozaenae and rhinoscleromatis (Orskov, I. 1984. Genus V. Klebsiella Trevisan 1885, 105. Krieg and Holt (editors) In Bergey's Manual of Systematic Bacteriology, 1:461-465). Klebsiella pneumoniae is the most common gram-negative pathogen causing community acquired pneumonia (Carpenter, J., et al, 1990. Rev Infect. Dis. 12:672-682). Klebsiella is also responsible for an estimated 8% of all nosocomial infections (Sahly, H. and Podschun, R., 1997. Clin. Diagn. Lab. Immunol. 4:393-399).
K. pneumoniae is an opportunistic pathogen that is associated with pneumonia, septicemia, meningitis, endocarditis, ventriculitis, and infections of urinary tract and wounds. These diseases are both nosocomial and community acquired. K. pneumoniae also plays a large role in two major nonrheumatoid arthritic diseases, Ankylosing Spondylitis and Reiter's Syndrome (Schwimmbeck, P. and Oldstone, M., 1989. Current Topics in Microbiology and Immunology. 145:45-56.). Despite available antibiotics, observed mortality rates for pneumonia are approximately 50%, but when bacteremic K. pneumoniae occurs in alcoholics, the mortality rises to almost 100% (Sahly, H. and Podschun, R., 1997. Clin. Diagn. Lab. Immunol. 4:393-399). The overall mortality rate for Klebsiella bacteremia in one study was 37% and has ranged in others from 25% to 55% (Watanakunakron, C. and Jura, J., 1991. Scand. J. Infect. Dis. 23:399-405).
Incidence of K. pneumoniae meningitis is on the rise. A study of 3377 cases of Bacterial meningitis in 1948, found only 7 were K. pneumoniae. In 1957, K. pneumonia accounted for 1.5% of all cases of meningitis. In an eleven-year study, from 1981 to 1991, 13% of culture proven bacterial meningitis cases were K. pneumoniae. There was an increase occurrence of K. pneumoniae meningitis within this study with 7% occurrence in the first 6 years, and 16% occurrence in the last 5 years (Tang, L-M and Chen, S-T., 1994. Scand. J. Infect. Dis. 26:95-102).
In recent years, Klebsiella strains have become multi-resistant to many antibiotics. In the 1970's, the resistance was mainly to aminoglycoside antibiotics. Since 1982, some Klebsiella strains have become resistant to the extended-spectrum cephalosporins (Sahly, H. and Podschun, R., 1997. Clin. Diagn. Lab. Immunol. 4:393-399). Resistance to the extended-spectrum cephalosporins among clinical isolates of Klebsiella in France and England has been reported at 14 to 16% (Sirot, D. 1995 J. Antimicrob. Chemother. 36:19-34). Since Klebsiella is a good recipient for R factors, resistance has been gained to β-lactams, tetracycline, chloramphenicols, ceftazidime, sulfonamides and trimethoprim. Today, almost all strains of Klebsiella are resistant to ampicillin. (Orskov, I. 1984. Genus V. Klebsiella Trevisan 1885, 105. Krieg and Holt (editors) In Bergey's Manual of Systematic Bacteriology, 1:461-465).
Microbial fermentation is an important way to convert renewable resources to products of biological and industrial importance. K. pneumoniae has been used to convert simple sugars to the commodity chemicals 1,3-propanediol and 1,2-propanediol. These products have been made by fed-batch fermentation of glycerol by K. pneumoniae. (Cameron, D. et al, 1998. Biotechnol. Prog. 14:116-125). Genes from the 1,3-propanediol pathway of K. pneumoniae have recently been cloned and expressed into both E. coli and S. cerevisiae. Metabolic engineering of these genes can significantly improve the product yield and productivity (Cameron, D. et al, 1998. Biotechnol. Prog. 14:116-125).
With K. pneumoniae playing the lead role, the Klebsiella genus is becoming an increasingly important pathogen. Over the past 10 years, discovery of multi-drug resistant strains has emphasized the importance of this genus. Furthermore, Klebsiella is considered to be a model for systemic infections caused by capsulated bacteria.
Vaccination is considered to be a very effective method of preventing infectious diseases in human and veterinary health care. Vaccination is the administration of immungenically effective amounts of antigenic material (the vaccine) to produce immunity to a disease/disease-causing pathogenic agent. Vaccines have contributed to the eradication of smallpox, the near eradication of polio, and the control of a variety of diseases, including rubella, measles, mumps, chickenpox, typhoid fever.
Before “the genomic era”, vaccines were based on killed or live attenuated, microorganisms, or parts purified from them. Subunit vaccines are considered as a modern upgrade of these types of vaccine, as the subunit vaccines contain one or more protective antigens, which are more or less the weak spot of the pathogen. Hence, in order to develop subunit vaccines, it is critical to identify the proteins, which are important for inducing protection and to eliminate others.
An antigen is said to be protective if it is able to induce protection from subsequent challenge by a disease-causing infectious agent in an appropriate animal model following immunization.
The empirical approach to subunit vaccine development, which includes several steps, begins with pathogen cultivation, followed by purification into components, and then testing of antigens for protection. Apart from being time and labour consuming, this approach has several limitations that can lead to failure. It is not possible to develop vaccines using this approach for microorganisms, which cannot easily be cultured and only allows for the identification of the antigens, which can be obtained in sufficient quantities. The empirical approach has a tendency to focus on the most abundant proteins, which in some cases are not immuno-protective. In other cases, the antigen expressed during in vivo infection is not expressed during in vitro cultivation. Furthermore, antigen discovery by use of the empirical approach demands an extreme amount of proteins in order to discover the protective antigens, which are like finding needles in the haystack. This renders it a very expensive approach, and it limits the vaccine development around diseases, which is caused by pathogens with a large genome or disease areas, which perform badly in a cost-effective perspective.